The present invention relates to composite membranes that include an inorganic cation exchange material, a polymer-based binder, and a silica-based binder. The composite membranes are suitable for use in electrochemical applications, particularly as membrane electrolytes in electrochemical fuel cells.
There is considerable demand for a high power density power source that is efficient, reliable, quiet, lightweight and environmentally friendly. Fuel cells, which are highly efficient electrochemical energy production devices, offer one potential solution. They convert chemical energy from renewable fuels directly into electrical energy.
Although the outlook for fuel cells is very promising, they have yet to achieve broad market penetration. The primary reason is that fuel cells require expensive materials and processes to manufacture. Proton exchange membrane (PEM) or solid polymer electrolyte (SPE) fuel cells currently represent one of the leading fuel cell technologies. PEM fuel cells have the highest power density of all the fuel cell designs. With suitable uses in the portable, stationary, and transportation markets, PEM fuel cells also offer broad market application. In addition, PEM fuel cells are extremely efficient (xcx9c50%), do not produce noise, and are relatively simple to manufacture.
PEM fuel cells typically employ an ion conducting electrolyte membrane between a positive electrode (cathode) and a negative electrode (anode), with each electrode containing a metal catalyst supported by a conductive material. Some PEM fuel cells use a diffusion layer on both electrodes to help distribute gases evenly across the electrode surfaces. Reactions take place where the electrolyte, gas, and electrode are in contact with one another.
The ion conducting membrane material plays a critical role in the operations of PEM fuel cells. It has three primary functions: (1) as an ion conductor between anode and cathode; (2) as a separator for the fuel and oxidant; and (3) as an insulator between the cathode and anode so that electrons conduct though an electronic circuit and not directly through the membrane.
During fuel cell operation, a fuel such as hydrogen is distributed evenly over the anode electrode. Hydrogen reacts with the catalyst to produce protons and electrons. The protons are transferred through the surface of the electrolyte to the cathode, while the electrons are conducted via an electrically conductive material through an external circuit to the anode. At the cathode, an oxidant such as oxygen permeates through the electrode where it combines with the electrons and protons to form water. The operation of PEM fuel cells produces three major byproducts: water, heat and electricity. The extent to which proton and electron resistance can be reduced in a PEM fuel cell, is a major factor in determining the future efficiency of this fuel cell design.
Several polymer electrolyte membranes have become the primary options for use as proton exchange membranes in PEM fuel cells. Unfortunately, these membranes have substantial cost and performance limitations that have hindered the commercialization of PEM fuel cells. Specifically, current PEM fuel cell membranes are very expensive to produce, and exhibit inadequate ionic conductivity, dehydration resistance, dimensional stability, and fuel crossover resistance.
Temperature tolerance is a fundamental requirement for a PEM fuel cell membrane. To make PEM fuel cells commercially viable, the membrane must be able to support operations at elevated temperatures. At these temperatures, power density is increased and sensitivity to fuel and oxidant impurities is reduced. High temperature operation also allows for cogeneration capabilities in fuel cell systems, which would broaden market appeal for these systems. High temperature operation is difficult to maintain using current ion exchange membranes, as they are prone to dehydration at elevated temperatures.
DuPont has developed a perfluoronated membrane under the trade name NAFION for use in fuel cells and other related applications. NAFION and its derivatives are the most commonly used ion exchange membranes in PEM fuel cells. NAFION, which is described, for example, in U.S. Pat. No. 4,330,654 is fabricated by melting tetrafuoroethylene and perfluorovinyl ethersulfonyl fluoride together, shaping the mixture, and then hydrolyzing the melt to yield the ionic sulfonate form.
While NAFION is an effective membrane within the context of PEM fuel cells, the polymer has a variety of limitations that have hampered the emergence of the PEM fuel cell design. NAFION conducts protons with the aid of water, but if the membrane is not properly hydrated, proton conduction slows. NAFION is also very susceptible to osmotic swelling.
There is a significant size difference between NAFION that is 0% hydrated and NAFION that is 100% hydrated, and this feature is a determinative factor in the longevity and performance of a PEM fuel cell. This osmotic swelling is of particular note in the cycle times of PEM fuel cells that use NAFION. If these fuel cells require many cycle times for operation, they will deteriorate faster than if they are continually in operation. This is primarily due to the swelling and shrinking of NAFION during PEM fuel cell cycles. As a corollary, the original state of the fuel cell is also altered mechanically during these cycles, thereby lowering PEM fuel cell performance over time.
The osmotic expansion of NAFION also reduces the consistency of the membrane electrode assembly (MEA) during production. In PEM fuel cells, catalyst is typically deposited directly onto the surface of NAFION using one of several techniques. The amount of deposited catalyst is critical to optimizing the cell design. Because of NAFION""s high osmotic expansion, the density of the catalyst may be affected at higher humidity levels.
NAFION also has inconsistent structures which, if not detected and removed, will limit fuel cell efficiency. These inconsistencies, which are referred to as chunks, result from the presence of inhomogeneous polymers. Chunks have lower proton conductivities which can adversely impact MEA and stack assembly.
High cost is another of NAFION""s major drawbacks. Due to its relatively complicated and time-consuming manufacturing process, NAFION is very expensive. A square meter of NAFION costs approximately $500. At this rate, the cost of NAFION represents a significant portion of the overall cost of a PEM fuel cell. Indeed, it may represent from 10 to 15% of the total cost of a single fuel cell or stack of fuel cells. It is generally accepted that if NAFION continues to represent the leading membrane candidate for PEM fuel cells, its cost must be reduced substantially before they can emerge as a competitive commercial alternative to existing power generators.
NAFION is also limited to operating temperatures below 100xc2x0 C. Among other things, high temperatures cause low proton conductivity, dehydration and degradation. Much research has focused on using sol-gel and other processes to infiltrate the porous structure of NAFION with components that will increase its performance at elevated temperatures. Staiti et. al. and Tazi et. al. impregnated NAFION with phosphotungstic acid and silicotungstic acid/thiophene, respectively, which increased proton conductivity and hydration levels at temperatures approaching 120xc2x0 C. See, P. Staiti, xe2x80x9cProton Conductive Membranes Based on Silicotungstic Acid/Silica and Polybenzimidazolexe2x80x9d, Materials Letters, 47, 2001, 241-246, and B. Tazi et al., xe2x80x9cParameters of PEM Fuel Cells Based on New Membranes Fabricated From Nafion, Silicotungstic Acid and Thiophenexe2x80x9d, Electrochimica Acta, 45, 2000, 4329-4339. Others including P. Costamagna et al. and Park et al. demonstrated that NAFION doped with zirconium hydrogen phosphate provided similar results as well. See, P. Costamagna et al., xe2x80x9cNafion 115/Zirconium Phosphate Composite Membranes for Operation of PEMFCs Above 100xc2x0 C., Electrochimica Acta, 47, 2002, 1023-1033 and Y. Park et al., xe2x80x9cProton Exchange Nanocomposite Membranes Based on 3-Glycidoxypropyltrimethoxysilane, Silicotungstic Acid and xcex1-Zirconium Phosphate Hydratexe2x80x9d, Solid State Ionics, 145, 2001, 149-160. However, by using NAFION as the base material, these membranes are still very expensive. In addition, these additives tend to leach out of the membrane structure during fuel cell operations, which limits their utility.
Developers are working on a variety of alternative membranes to resolve the technical limitations facing NAFION in PEM fuel cells, but none of these alternatives has demonstrated sufficient advantages to replace NAFION as the membrane of choice. One alternative membrane incorporates NAFION or a NAFION-like polymer into a porous polytetrafluoroethylene (TEFLON) structure. These membranes are available under the trade name GORE-SELECT from W. L. Gore and Associates, Inc. and they are described in U.S. Pat. Nos. 5,635,041, 5,547,551 and 5,599,614. Other alternative membranes are available under the trade names ACIPLEX from Asahi Chemical Co. and FLEMION from Asahi Glass. Due to their polyfluorinated structures, these alternative membranes exhibit many of the same deficiencies as NAFION, namely, inadequacies with respect to ionic conductivity, dehydration resistance, dimensional stability and fuel crossover.
Composite ion exchange membranes containing a low expansion, durable polymer impregnated with a high proton conductive polymer described, for example, in U.S. Pat. No. 6,248,469, represent another alternative. The main disadvantage of these membranes is that they loose the cross sectional area of the high proton conductive material due to the presence of the durable polymer support.
A further alternative membrane employs polybenzimidazole polymers (PBI) that are infiltrated with phosphoric acid. These have been used as ion exchange membranes in PEM fuel cell,s and are described in U.S. Pat. Nos. 5,716,727 and 6,099,988. These membranes permit PEM fuel cells to operate at higher temperatures of about 130xc2x0 C., and exhibit lower osmotic expansion than that of NAFION. However, the concentrated acid in the PBI pores leaches out as water is produced from the electrochemical fuel cell process, thereby dramatically reducing membrane and electrochemical cell performance. The leached phosphoric acid may also react poorly with other components in the fuel cell stack.
Finally, more recent research has led to unique formulations and designs of ion exchange membranes. For example, Chen et. al. showed that incorporating montmorillonite and lithium triflate into poly(ethyl oxide) (PEO) produced a membrane that exhibited fuel cell output that was nearly sixteen times higher than in fuel cells using PEO by itself. See Chen et al., xe2x80x9cThe Novel Polymer Electrolyte Nanocomposite Composed of Poly(ethylene oxide), Lithium Triflate and Mineral Clayxe2x80x9d, Polymer, 42, 2001, 9763-9769. However, the increased proton conductivity values in these fuel cells were still substantially lower than that produced by fuel cells using NAFION. Similarly, Aranda et. al. created a membrane by combining poly(ethylene oxide) and ammonium exchanged montmorillonite, but the membrane also exhibited low ion conductivity.
As is apparent, there is a need for an inexpensive and higher performing proton exchange membrane for use in PEM and other low operation temperature fuel cells, and for one or more methods of membrane fabrication that are more cost effective than those used to produce NAFION.
The present invention is based, in part, on the discovery of composite electrolyte membranes that can be used as proton exchange membranes in PEM fuel cells, and the processes for producing these membranes.
One aspect of the invention is directed to a composite electrolyte for use in electrochemical fuel cells that includes:
(i) an inorganic cation exchange material;
(ii) a silica-based binder; and
(iii) a polymer-based binder.
Preferred cation exchange materials include, for example, clays, zeolites, hydrous oxides, and inorganic salts.
In another aspect, the invention is directed to in an electrochemical fuel cell that includes:
(i) an anode;
(ii) a cathode;
(iii) fuel supply means for supplying fuel toward the anode;
(iv) oxidant supply means for supplying an oxidant toward the cathode; and
(v) a composite electrolyte as defined above that is positioned between the anode and cathode.
In a further aspect, the invention is directed towards a method of fabricating a composite membrane suitable for use in an electrochemical fuel cell that includes the steps of:
(i) applying a viscous liquid composition comprised of (a) an inorganic cation exchange material, (b) a silica-based binder, (c) a polymer-based binder, and (d) a solvent onto a surface of a substrate;
(ii) spreading the viscous liquid composition to form a uniform thickness layer on the substrate; and
(iii) allowing the solvent to evaporate from the viscous liquid composition to yield the composite electrolyte.